Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	$ 1,098.3
Other comprehensive loss before reclassifications	50.1	$ (154.4)
Amounts reclassified from AOCI	1.9	1.8
Other comprehensive income (loss)	52.0	(152.6)
Ending balance	1,139.2	1,351.5
Pretax reclassification amounts from AOCI, tax	(0.5)	(0.5)
AOCI
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(399.4)	(50.0)
Ending balance	(347.4)	(202.6)
Net Unrealized Investment Gains (Losses) on Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(449.6)	347.1
Other comprehensive loss before reclassifications	91.3	(335.9)
Amounts reclassified from AOCI	1.9	1.8
Other comprehensive income (loss)	93.2	(334.1)
Ending balance	(356.4)	13.0
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(2.4)	(2.3)
Net Reserve Remeasurements Attributable to Discount Rates
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	59.0	(386.9)
Other comprehensive loss before reclassifications	(41.2)	181.5
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income (loss)	(41.2)	181.5
Ending balance	17.8	(205.4)
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(8.8)	(10.2)
Other comprehensive loss before reclassifications	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income (loss)	0.0	0.0
Ending balance	$ (8.8)	$ (10.2)